It was a ruling by the Sage that no matter how many slam/unarmed attacks a level-draining creature has, it can only use one attack to drain each round. A monk vampire could still use their iterative attacks, but they'd only be able to inflict their negative levels once in any one round. That's still 2 negative levels per round though, which makes it progressively more difficult to fight.
I'll try to find where this ruling was, I can't remember off-hand where it was made, or if it was ever made "official."
I'll try to find where this ruling was, I can't remember off-hand where it was made, or if it was ever made "official."